silt up vs silt

silt up is not a noun while silt is a noun.

Word	Noun	Adjective	Verb	Adverb
silt up	No	No	Yes	No
silt	Yes	No	Yes	No

As verbs, silt up and silt are synonyms defined as:

silt up (noun)	silt (noun)
	mud or clay or small rocks deposited by a river or lake

silt up (verb)	silt (verb)
become chocked with silt	become chocked with silt